The present invention relates generally to ultrasonic imaging catheters, and more particularly, to improved transducers for use in ultrasonic imaging catheters.
Intravascular imaging of blood vessels and surrounding tissues continues to be of great benefit in a wide range of medical fields. A particularly successful design for an intravascular imaging catheter employs a rotatable imaging assembly containing an ultrasonic transducer, where the assembly is attached to the distal end of a flexible drive cable. The transducer may be rotated within a catheter body or sheath in order to transmit an ultrasonic signal and produce a video image by well-known techniques. The transducer element or elements are connected to electronics, typically maintained outside the patient""s body, to produce the video image.
When a sound wave generated by a typical transducer impinges on an interface between two different media, such as the interface between the transducer face and the tissue being imaged, part of the incident wave is reflected and part is transmitted. The amount of wave reflected compared to the amount transmitted depends primarily on the relative acoustic impedances of the two media at the media interface. For some transducers, this difference can be quite large. For example, the acoustic impedance of a atypical piezoelectric transducer is about 30 mRayls, and the acoustic impedance of tissue is about 3 mRayls. In general, it is desirable to reduce or minimize the difference in acoustic impedance between the two media to permit a greater amount of the ultrasound wave to transmit through the interface.
In order to reduce the impedance mismatch between the transducer and tissue, some existing catheters attach one or more matching layers to the transducer face which have an acoustic impedance between that of the transducer and that of the tissue being imaged. In general, having a greater number of interfaces, each with a small acoustic impedance mismatch, is more desirable than a single interface having a large impedance mismatch.
Another technique involves using transducers made from piezocomposite material. In these transducers, the piezoelectric material is mixed with non-piezoelectric material to reduce the transducer""s overall acoustic impedance. For example, as shown in FIG. 1, a prior art transducer 200 has a plurality of columns 210 made from piezoelectric material interspersed with a plurality of columns 220 made from non-piezoelectric material.
While this transducer has achieved some degree of success, it is desirable to provide a piezoelectric transducer having a better acoustic impedance profile to provide better acoustic impedance matching with tissue or the matching layer. It is further desirable to provide impedance matching without greatly increasing the number of interfaces the ultrasound signals must cross.
The present invention provides improved ultrasound transducers, transducer packages, and methods of making same. The transducer packages of the present invention are intended to overcome at least some of the problems of the prior art, and will be particularly useful for ultrasound imaging catheters. For example, transducer elements and packages of the present invention are designed to reduce the acoustic impedance at the imaging surface of the transducer. Such transducers hence provide better acoustic impedance matching, and have improved performance.
In one embodiment, the present invention provides an exemplary transducer element for use in an imaging catheter. The transducer element has first and second transducer surfaces defining a thickness therebetween. The transducer includes a plurality of tapered pillars that comprise piezoelectric material and extend between the first and second transducer surfaces. At least one of the pillars has a first cross-sectional area at the first transducer surface that is larger than a second cross-sectional area at the second transducer surface. In this manner, the pillar has an increasingly smaller cross-sectional area as it tapers away from the first transducer surface.
In one aspect, the transducer element further includes a backing material operably attached to the first transducer surface. Similarly, in one aspect the transducer element further includes a matching layer operably attached to the second transducer surface.
In one particular aspect, the transducer element further includes a filler material disposed between the pillars and defining a portion of the second transducer surface. In one aspect, the filler material also defines a portion of the first transducer surface. Alternatively, the plurality of pillars merge together to completely define the first transducer surface. In this manner, the first transducer surface is completely defined by piezoelectric material.
Preferably, the filler material is selected from a group of materials consisting essentially of epoxy, gel, plastic, air, combinations of such materials such as epoxy with air bubbles, and the like. Such filler materials have a lower acoustic impedance than an acoustic impedance of the pillars.
In one aspect, the first cross-sectional area of at least one of the pillars has a shape that is generally rectangular. In another aspect, the first cross-sectional area of at least one of the pillars has a shape selected from a group of shapes consisting of a square, a rectangle, a circle, an ellipse and an oval. Preferably, at least one of the pillars has a sloped outer surface that is positioned at a non-perpendicular angle to the second transducer surface. In this manner, the pillar tapers away from the first transducer surface and has a smaller cross-sectional area further from the first transducer surface.
In another embodiment, the present invention provides a transducer element having a base which defines a first transducer surface. The transducer element includes a plurality of columns extending from the base. The columns comprise piezoelectric material, and each column has an upper surface. The upper surfaces of the columns collectively define a first portion of a second transducer surface. At least one of the columns has a first cross-sectional area at the base that is larger than a second cross-sectional area at the second transducer surface.
In one aspect, the transducer element further includes a filler material disposed between the plurality of columns and defining a second portion of the second transducer surface. Preferably, the second portion of the second transducer surface is larger than the first portion. In this manner, the second transducer surface is defined by more filler material than column material. In one aspect, the transducer has a first acoustic impedance at the base that is greater than a second acoustic impedance at the second transducer surface. In one particular aspect, the base includes a piezoelectric material, such as a piezoplastic, piezoceramic and the like.
The present invention further provides a transducer package for use in an imaging catheter. The transducer package includes a transducer having a base. The base defines a first transducer surface. A plurality of pillars extend from the base and comprise piezoelectric material. Each of the pillars has an upper surface, with the upper surfaces collectively defining a first portion of a second transducer surface. At least one of the pillars has a first cross-sectional area at the base that is larger than a second cross-sectional area at the upper surface. The transducer package further includes a backing material operably attached to the first transducer surface.
In one aspect, the transducer package further includes a filler material disposed between the pillars and defining a second portion of the second transducer surface. Together, the pillar upper surfaces and filler material completely define the second transducer surface.
The present invention further provides methods of making transducers and transducer packages, particularly for use in imaging catheters. In one particular embodiment, a method of the present invention includes the steps of providing a transducer element having first and second spaced apart surfaces defining a transducer element thickness therebetween. The transducer element comprises piezoelectric material having a first acoustic impedance. The method includes removing a portion of the transducer element to create a plurality of pillars extending between the first and second surfaces. At least one of the pillars has a first cross-sectional area at the first surface that is larger than a second cross-sectional area at the second surface. The method includes placing a filler material between the plurality of pillars. The filler material has a second acoustic impedance that is less than the first acoustic impedance. In this manner, the second surface is made up of more filler material than is the first surface. As a result, the second surface has a lower acoustic impedance than the first surface.
In one aspect, the plurality of pillars merge together to completely define the first surface. In one particular aspect, the removing step includes cutting a portion of the transducer element with a cutting apparatus and removing that portion. In one aspect, the removing step creates at least one of the pillars to be a tapered pillar. The tapered pillar has a cross-sectional area that increases as the tapered pillar extends away from the second surface. Alternatively, the plurality of pillars comprises a plurality of tapered pillars.
In one aspect, the removing step creates the plurality of pillars to have a stair-step tapered shape. In another aspect, the removing step creates a plurality of gaps at the first surface between the plurality of pillars. In this manner, the filler material defines part of the first surface.
In one aspect, the method further includes the step of mounting a backing material to the first transducer surface. Preferably, the backing material is a sound-attenuating material. In one aspect, the mounting step occurs prior to the removing step. For example, mounting the backing material to the first transducer surface before the removing step may be desirable when the removing step will create gaps at the first surface between the plurality of pillars.
In another method of the present invention, a transducer element is provided which includes piezoelectric material having a first acoustic impedance. A portion of the transducer element is removed to create a base portion of the transducer element and a plurality of pillars extending from the base portion. The base portion defines a first transducer surface. The plurality of pillars each have an upper surface, and at least one of the pillars has a first cross-sectional area at the base portion that is larger than a second cross-sectional area at the upper surface. The method includes adhering a filler material between the plurality of pillars. The filler material has a second acoustic impedance that is lower than the first acoustic impedance. The filler material and the plurality of pillar upper surfaces define a second transducer surface.
In still another method of the present invention, a piezoelectric material is provided and formed into a desired shape having a base portion and a plurality of pillars. In one aspect, the forming step includes molding the piezoelectric material. This can be accomplished, for example, by injection molding, press molding, casting, and the like.
Other features and advantages of the invention will appear from the following description in which the preferred embodiment has been set forth in detail in conjunction with the accompanying drawings.